1. Field of the Invention
The invention pertains to photoelectric apparatus for counting interface gaps between spaced storage discs in a rotary file of multiple flexible or floppy type magnetic discs in continuous motion; especially when the disc edges are subject to transient fluttering parallel to the axis of rotation.
2. Cross References to Related Applications
1. U.S. patent application Ser. No. 375,985 by R. O. Cobb et al, filed July 2, 1973 now U.S. Pat. No. 3,835,998.
2. U.S. patent application Ser. No. 414,614 by R. J. Penfold et al, filed Nov. 7, 1973, now U.S. Pat. No. 3,867,723.
3. Description of the Prior Art
The above cross-referenced Penfold et al application discloses a rotating magnetic disc storage file, including multiple flexible or floppy storage discs maintained in continuous high speed rotation, wherein successive discs are separated by relatively small diameter thin spacer discs. Typically the storage discs have nominal thickness of about 0.003 inch. Outward application of pressured air, through the longitudinally apertured spindle which supports the discs, causes the storage discs to be spaced uniformly at intervals of about 0.003 inch when rotating at typical speed of 1800 rpm.
Spaced disc files of this type represent improvement, in terms of design simplicity, operational stability and access time efficiency, over flexible disc files of the kind disclosed in U.S. Pat. Nos. 3,509,533 (Krijnen) and 3,618,055 (Van Acker). In turn the latter files represent ostensible evolutionary advancements over an earlier type of non-flexible (or semi-flexible) disc file disclosed in U.S. Pat. No. 3,130,393 (Gutterman). In the Gutterman patent relatively thicker slidably mounted magnetic discs rotate contiguously in a closed air-tight environment and are separated for access by application of high pressure air radially inward at a selected interface. This displaces a subset of the discs, in a piston-like operation, to form a space (gap) suitable for accommodating a magnetic head.
In files of stacked flexible discs it is usually desirable to locate the access position by sensing and counting operations. However such sensing and counting must not be subject to error when there is fluttering of the discs or run-out variations in disc thickness which could cause double sensing (false counting) of a gap.
The Gutterman patent specifically suggests cumulative electronic counting of disc edges sensed by electromechanical (piezoelectric) means and inplies alternative use of magnetic or optical edge sensing.
The above cross-referenced patent application by R. O. Cobb et al addresses problems incidental to tracking axial components of flexible disc motion by parallel optical sensing. It discloses a stationary array of integrated circuit photodetectors which is light coupled in parallel to all of the discs. Individual photodetectors of the array have width dimensions an order of magnitude smaller than the nominal disc thickness so that several detector elements are light coupled to each disc. By sensing the distribution of light across the detector elements the position of the access apparatus relative to the discs is instantaneously distinguishable for parallel sampling and electronic counting. With large numbers of discs this arrangement can be quite costly, at the present state of development of integrated circuit technology, and dissipates more power than the serial sensing arrangement suggested by Gutterman which would use a single sensor coupled to only one disc interface at a time.
However, in sensing the discs one at a time by optical coupling, light must either be confined to one disc edge and sensed upon reflection or confined to one interface space (gap) and sensed after transmission. Realization of this under fast access conditions with a high degree of reliability is not simple to achieve. For reflective coupling optimal results are realized only if the disc edges are processed to a smooth reflective finish (e.g., by lathe trimming, polishing, painting, etc.) and this naturally increases system fabrication and maintenance costs. On the other hand transmissive coupling through inter-disc spaces presents difficulties due to the narrow width (about .003 inch and long length of the coupling path (several inches in a file of 12 inch diameter discs). Light attenuation due to dispersion and scattering (e.g., from dust, debris on the disc surfaces, and/or edge slivers on the discs) weakens the coupling and thereby increases the possibility of erroneous readings. Upon error the access assembly must be repositioned degrading access time performance.
With either type of coupling (reflective or transmissive) transient light-to-dark transitions due to transitory components of disc edge motion (due to fluttering and/or variations in disc shape or thickness) can be quite difficult to track and accurately count.
Transmissive coupling losses can be reduced by use of coherent (i.e., laser) light but the increased cost may be unattractive. Another obvious alternative, increasing the intensity of non-coherent source light, presents the risk of subjecting the recording surfaces of the discs to possible damage or warping stress due to the accompanying heat.